Automotive safety belt retractors having an inertia actuated latch against belt extraction conventionally employ a pivoted latch bar for engaging the teeth of a pair of one-way ratchet sprocket gears actuated to engaging position by an inertia element responsive to vehicle deceleration. The latch bar is in a normally disengaged position and must be raised through a substantial arc to reach a fully engaged ratchet tooth position.
In order to minimize the size and stroke of a suspended pendulum or other type of inertia element for actuating the latch bar, an auxilliary sprocket gear is normally employed in fixed concentric relation to one of the dual ratchet gears with an equal number of elongated teeth adapted for preliminary engagement by initial pivotal movement of a lever produced by the inertia element when initially displaced, such lever in turn raising the latch bar to its ratchet gear tooth engaging position. The extended tooth form is adapted to produce additional arcuate lever actuation sufficient to pivot the latch bar into full latch engagement.
The relative geometry of the pivoted lever having a narrow finger for engaging the elongated auxilliary tooth is such as to produce a radial sliding of the lever finger contact point radially inward along the elongated tooth which multiplies the angular displacement of the lever, and corresponding displacement of the latch bar, relative to the angular travel of the elongated tooth as required to produce full engagement of the latch bar with minimal lost motion following pendulum displacement. The conventional prior art form of the elongated auxilliary gear tooth has been a narrow straight tooth form with the engagement face in radial alignment with the gear axis. The narrow finger of the lever arm has an elongated tapered finger form terminating in a sharp point adapted to initially engage the actuating side of a sharp point of the elongated tooth.
In order to maximize the ratio of arcuate travel of the lever arm and latch bar relative to the sprocket gears, and thereby minimize lost motion to produce belt locking against extraction, it is desirable for the angle between the line passing through the lever pivot and its finger point of initial contact with the elongated tooth extremity and a line normal to the initial engaging contact surface of such tooth be as small as possible consistent with free-sliding contact of the lever engagement point with the elongated tooth surface following the initial engagement. The effective "angle of friction" for the plastic materials involved defines a limit for reducing such angle since any possible sticking upon initial contact may cause fracturing of the tooth or lever finger, or both. Moreover, even if a sliding angle is provided relative to the effective "angle of friction" of the plastic parts, an impact stress may be involved at the contact points proportional not only to suddenness of belt extraction and retraction spool acceleration, but also proportional to the impact force vector which is ineffective to produce lever pivoting. Repeated impact of the tips at an angle close to the "angle of friction" may dull the points and enhance the possibility of tip engagement at a jamming angle. In practice a 14.degree. design-angle has been employed on the assumption that it provides an adequate margin of safety for assuring sliding upon initial contact as well as an acceptable rate of latch bar engagement; however, due to impact of contact points, tolerance extremities, irregularities including wear at the contact points, and sliding tooth surface, as well as other possible unknown factors, jamming of the contact points, and fracturing of the tooth or finger element, or both have been found to occur occassionally resulting in malfunction and service requirement.